(a) Field of the Invention
The invention relates to improvements in a free turbine type engine. More specifically, the invention relates to such an engine including a power limiter unit.
(b) Description of Prior Art
It is known in the art to provide aircraft engines wherein temperature and pressure conditions are measured at different parts thereof. The data obtained from the measurement is used to calculate control signal levels for providing input to a control means for controlling different parameters of the engine. Such engines are illustrated in, for example, U.S. Pat. Nos. 2,808,702, D. A. Dotson, issued Oct. 8, 1957, 2,910,125, S. G. Best, issued Oct. 27, 1959, 3,019,604, W. M. Hall, issued Feb. 6, 1962, 3,420,056, K. Eames, issued Jan. 7, 1969, 3,421,317, A. Bedford, issued Jan. 14, 1969, 3,971,208, G. V. Schwent issued July 27, 1976, 4,275,557, I. E. Marvin et al, issued June 30, 1981, 2,667,228, Wood et al issued Jan. 26, 1954, 2,734,340, C. F. Wood, issued Feb. 14, 1956, 2,977,756, M. C. Stone, issued Apr. 4, 1961, 3,031,840, B. S. Hegg et al, issued May 1, 1962, 3,203,179, B. E. Blackaby, issued Aug. 31, 1965, 3,596,467, P. A. Avery, issued Aug. 3, 1971, 3,620,011, L. A. Urban et al, issued Nov. 16, 1971, 4,159,625, W. B. Kerr, issued July 3, 1979, 4,161,101, T. M. Drummond, issued July 17, 1979, 4,185,460, Moore et al, issued Jan. 29, 1980 and 4,242,864, Cornett et al, issued Jan. 6, 1981.
It is also known in the art to compare the calculated level with a reference level, and to provide the difference of the comparison to the input of the control means. Such an engine is illustrated in U.S. Pat. No. 3,420,056.
The general ideas above discussed are, thus, well known in the art. However, as can be seen from the study of the above references, improvements are possible in the specific locations for measuring temperature and pressure, in the specific mathematical manipulations to which the measured data is applied, and in the specific parameter which is then controlled by the calculated value.
It is also known that the traditional way of measuring power of turbo prop and turbo shaft engines is to measure the torque of the output shaft. This is accomplished by measuring the twist of the shaft or, in some cases, the torque is determined by means of measuring the reaction of a helical ring gear that has its axial reactive mode opposed by a hydraulic cylinder. The level of pressure in the cylinder is directly proportional to the torque being fed through the gear box. The torque pressure is converted to an electrical signal that is properly conditioned to indicate torque on a suitable instrument on a control panel.
Although the latter system has proven to be reliable, it fails in fully aerobatic installations when the aircraft is flown in negative "g" conditions, i.e., upside down. This manoeuver will result in momentary loss or reduction of engine oil pressure, causing the hydraulic torque signal to fall below the true value, thus resulting in power fluctuations in installations which incorporate torque limiters or torque controllers.
In order to overcome this difficulty, it is necessary to provide an engine wherein the control signal for power or torque limiting is provided without measuring the power or torque directly.